1. Field of the Invention
The present invention relates to an image heating apparatus configured to heat a recording medium at a nip portion between a heating member having an elastic layer and a conveying member, and more specifically to a layer structure of the heating member permitting lowering of a target temperature in temperature control of an outer surface temperature of the heating member without hampering the performance thereof in heating the recording medium.
2. Description of the Related Art
An image forming apparatus configured to transfer a toner image carried on an image carrier to a recording medium and to fix an image on the recording medium by heating and pressing the recording medium on which the toner image has been transferred at a nip portion of a fixing apparatus, i.e., one exemplary image heating apparatus, is being widely used. The image heating apparatus has the nip portion for the recording medium formed by making a conveying member (a roller member or a belt member) come into contact with the heating member (a roller member or a belt member). The heating member is provided with an elastic layer having rubbery elasticity on a base layer (a cylindrical member or a belt member) bearing the strength of the heating member to enhance followability thereof on an uneven surface of the recording medium.
Japanese Patent Application Laid-open No. 2007-219371 enhances the thermal conductivity of a fixing belt in a thickness direction thereof by blending oxide metallic thermal conductive fillers, such as alumina and silica, into a silicone rubber material forming an elastic layer. Japanese Patent Application Laid-open No. 2005-302691 provides a fluorine resin release layer having high releasability to melted toner on an elastic layer and enhances thermal conductivity of the release layer by blending metallic thermal conductive fillers, such as gold and nickel, into the fluorine resin material of the release layer.
If the quality of a heat-processed image and the heat processing speed are same in the image heating apparatus, it is desirable to be able to lower the outer surface temperature of the heating member. The lower the outer surface temperature of the heating member, the less the heat radiated from the whole surface, so that the power required to maintain the outer surface temperature of the heating member can be saved. The lower the outer surface temperature of the heating member, the less the wear rate of the release layer on the surface of the heating member, so that the replacement life of the heating member can be also prolonged.
It was confirmed that it is possible to lower the outer surface temperature of the heating member by lowering a target temperature in temperature control in a case where the thermal conductivity of the release layer is increased, as disclosed in Japanese Patent Application Laid-open No. 2005-302691. However, its effect cannot be said to be sufficient by the thickness of the release layer disclosed in Japanese Patent Application Laid-open No. 2005-302691, and it is necessary to increase the target temperature by a certain degree in the temperature control in order to assure the quality of a heat-processed image and the heat processing speed. Accordingly, it is unable to fully lower the outer surface temperature of the heating member.